In the MRI apparatus, when a subject is in motion while imaging is performed, such motion may exert an influence on all over the image, and cause an artifact (hereinafter, it is briefly referred to as “motion artifact”) which looks like a flowing of an image in the phase encoding direction. Such artifact occurs, because when echo signals of respective lattice points in a data space, which is generally called as “k-space”, are sampled, the sampling parallel to a frequency encoding direction is repeated in the phase encoding direction (hereinafter, referred to as “Cartesian sampling method”).
As against this Cartesian sampling method, there is a technique referred to as “Non-Cartesian sampling method”. Examples of this Non-Cartesian sampling method may include radial sampling method (non-patent document 1), and hybrid radial method (non-patent document 2, referred to as “propeller MRI method” in this document) that combines the phase encoding with the radial sampling method.
The radial sampling method is a technique in which sampling is conducted radially, while a rotation angle is changed, setting approximately one point (generally original point) in the k-space as the rotation center, thereby obtaining echo signals which are required for reconstructing one piece of image. When imaging is performed by using the radial sampling method, the motion artifacts spread out peripherally on the image (i.e., they go out of a focused FOV), since the sampling is conducted radially. Therefore, it is said that compared to an imaging by the Cartesian sampling method, the radial sampling method makes the motion artifact inconspicuous and it is robust against the body motion.
However, the radial sampling method acquires the k-space radially, and therefore, a distribution of a readout gradient magnetic field within an imaging plane varies with respect to each echo signal. Therefore, if there are inhomogeneities in the magnetic field and/or nonlinearity in the gradient magnetic field, an influence therefrom varies from one echo signal to another. In other words, a gradient magnetic field to be applied in executing a sequence, which is calculated based on the coordinates in the k-space, is different from the gradient magnetic field actually applied in acquiring an echo signal, and therefore, the echo signal cannot be placed on a proper coordinate within the k-space. Given the situation above, compared to the Cartesian sampling method, there is found a problem that the radial sampling method may easily generate an artifact in the image, due to the nonlinearity of the gradient magnetic field and/or the magnetic field inhomogeneities. In order to correct the artifact in the radial sampling method, there is a technique to measure in advance the nonlinearity of the gradient magnetic field before executing the sequence, and the result of the measurement is reflected in a real measurement (non-patent document 3).
[Non-Patent Document 1]
G. H. Glover et al, Projection Reconstruction Techniques for Reduction of Motion Effects in MRI, Magnetic Resonance in Medicine 28: 275-289 (1992)
[Non-Patent Document 2]
James G. Pipe, Motion Correction With PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging, Magnetic Resonance in Medicine 42: 963-969 (1999)
[Non-patent document 3]
D. C. Peters et al, Centering the Projection Reconstruction Trajectory: Reducing Gradient Delay Errors, Magnetic Resonance in Medicine 50: 1-6 (2003)